Glass transition dynamics and structural relaxation of PLLA studied by DSC : Influence of crystallinity

摘要

Poly(L-lactic acid), PLLA, in amorphous and semi-crystalline forms were studied by DSC, in order to investigate both molecular dynamicsand structural relaxation features, and to understand the influence of the crystalline confinement on the segmental mobility of the intraspheruliticamorphous phase. Experimental data were generated after submitting the materials to different thermal histories below Tg andwere treated with a model based on the configurational entropy concept, allowing to extract a series of physical-meaningful parameters and toobtain the temperature dependence of the relaxation times. The main features of the relaxation process in the glassy state (activation energyof the glassy state and x) and the fragility index were found to be apparently insensitive to crystallinity. Significant differences between thetwo materials were detected in both the position of the glass transition temperature and the width of the distribution of relaxation times. In theframework of the Adam and Gibbs theory, it is suggested that for the semi-cristalline PLLA the mobility of the amorphous chains is more orless restricted, depending on their distance to the rigid lamellar walls or on the intra-lamellar thickness; this will imply that theirconformational motions will take place at different temperatures, typically above the glass transition temperature of the (unconfined) bulkamorphous phase.